Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (OEL) devices, have numerous well-known advantages over other flat-panel display devices currently in the market place. Among the potential advantages is brightness of light emission, relatively wide viewing angle, reduced device thickness, and reduced electrical power consumption compared to, for example, backlight displays.
Applications of OLED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The organic EL medium structure can be formed of a stack of sub-layers that can include small molecule layers or polymer layers. Such organic layers and sub-layers are well known and understood by those skilled in the OLED art.
Full-color OLED devices may employ a variety of organic materials to emit different colors of light. In this arrangement, the OLED device is patterned with different sets of organic materials, each set of organic materials associated with a particular color of light emitted. Each pixel in an active-matrix full-color OLED device typically employs each set of organic materials, for example to form a red, green, and blue sub-pixel. The patterning is typically done by evaporating layers of organic materials through a mask. In an alternative arrangement, a single set of organic materials emitting broadband light may be deposited in continuous layers with arrays of differently colored filters employed to create a full-color OLED device.
The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device. The present invention may be directed to either a top-emitting or bottom-emitting OLED device. In top-emitting OLED devices, light is emitted through an upper electrode or top electrode, typically but not necessarily the cathode, which has to be sufficiently light transmissive, while the lower electrode(s) or bottom electrode(s), typically but not necessarily the anode, can be made of relatively thick and electrically conductive metal compositions which can be optically opaque. Because light is emitted through an electrode, it is important that the electrode through which light is emitted be sufficiently light transmissive to avoid absorbing the emitted light. Typical prior-art materials proposed for such electrodes include indium tin oxide (ITO) and very thin layers of metal, for example silver, aluminum, magnesium or metal alloys including these metals.
A variety of circuits for controlling conventional, single-OLED devices are known in the prior art. For example, referring to FIG. 6, an OLED 120 is driven by a circuit comprising a drive transistor 110 in response to a data signal 130 stored in a capacitor 102 and deposited through a deposition transistor 100 by data and select control signals 140 and 150, respectively. The circuits typically comprise thin-film silicon materials, for example amorphous silicon or low-temperature polysilicon, as is known in the art. More complex circuits compensating for deficiencies in the materials or manufacturing are also known. However, such circuits are not suitable for driving stacked OLED devices.
Stacked OLED devices are employed to improve the resolution and reduce the current density in the OLED material layer, thereby improving lifetime. One approach to dealing with the aging problem, while maintaining the resolution of the display, is to stack two or more OLED light-emitting elements on top of each other thereby allowing the areas of the light-emitting elements to be larger to improve lifetime, and/or allowing more pixels to be provided for a given area, thereby improving resolution. This approach is described in U.S. Pat. No. 5,703,436 by Forrest et al., issued Dec. 30, 1997, and U.S. Pat. No. 6,274,980 by Burrows et al., issued Aug. 14, 2001. Stacked OLEDs utilize a stack of light-emitting elements located one above another over a substrate. Each light-emitting element may share one or both electrodes with a neighboring light emitting element in the stack and each electrode is individually connected to an external power source, thereby enabling individual control of each light-emitting element. However, forming such structures is difficult and, especially, providing external electrode connections may be problematic.
Referring to FIG. 5, a stacked OLED device is illustrated having a substrate 10 (either reflective, transparent, or opaque). Over the substrate 10, a first electrode 50 is formed. A first light-emitting layer 52 is formed over the first electrode 50 and a second electrode 54 formed over the first light-emitting layer 52. The first and second electrodes 50 and 54 provide current to the first light-emitting layer 52 through separate power connections 58. An insulating layer 56 may be provided over the second electrode 54 to isolate it electrically from the third electrode 60 formed over the insulating layer 56. A second light-emitting layer 62 is formed over the third electrode 60 and a fourth electrode 64 formed over the second light-emitting layer 62. The third and fourth electrodes 60 and 64 provide current to the second light-emitting layer 62 through separate power connections 68. An insulating layer 66 may be provided over the fourth electrode 64 to isolate it electrically from the fifth electrode 70 formed over the insulating layer 66. A third light-emitting layer 72 is formed over the fifth electrode 70 and a sixth electrode 74 formed over the third light-emitting layer 72. The fifth and sixth electrodes 70, 74, respectively, provide current to the third light-emitting layer 72 through separate power connections 78. The separate power connections 58, 68, 78 may be provided to independently control each of the first, second, and third light-emitting layers 52, 62, 72. The first, second, and third light-emitting layers 52, 62, 72 may emit three colors of light, for example red, green, and blue to form a full-color device.
U.S. Pat. No. 6,844,957 B2 entitled, “Three Level Stacked Reflective Display”, issued Jun. 7, 2005 to Matsumoto et al., describes a structure and fabrication technology for a reflective, ambient light, low cost display incorporating a plurality of cells laid out side by side and stacked as many as three levels on top of each other. Each stack of three cells driven by an array of TFT's is positioned on the bottom layer. Each cell comprises a light transmitting front window, three levels of individual cells RGB (Red, Green, and Blue) stacked on top of each other, each level having its own individual electrode, each electrode being connected by vertical conducting via holes running through each transparent dielectric spacer and being connected to an individual TFT. However, control circuits are not provided.
Alternative devices employing stacked light-emitting layers and color filters are also known. Commonly assigned, co-pending U.S. application Ser. No. 11/087,522, entitled “OLED Display Device,” filed Mar. 23, 2005 by Miller et al., which is hereby incorporated in its entirety by reference, describes a full-color OLED display device comprised of a substrate; an array of light-emitting elements, each element comprised of a first EL unit positioned between and in electrical contact with a first pair of electrodes and a second EL unit positioned between and in electrical contact with a second pair of electrodes and located above or below the first EL unit, wherein the first EL unit emits light primarily in only two of the red, green and blue portions of the visible spectrum, and the second EL unit emits light at least in the remaining third portion of the visible spectrum; and a means for selectively filtering the light produced by the first EL unit to filter light from one of the only two of the red, green and blue portions of the visible spectrum in some light-emitting elements and to filter light from the other of the only two of the red, green and blue portions of the visible spectrum in some other light-emitting elements. The disclosure includes a drive circuit, but this circuit is only useful for stacks having three independent electrodes. As noted above, it is difficult to construct these stacks.
U.S. Pat. No. 6,903,378 entitled, “Stacked OLED Display Having Improved Efficiency” by Cok, issued Jun. 7, 2005, which is hereby incorporated in its entirety by reference, describes an OLED device having a pixel, including a plurality of light transmissive filters; a first electrode layer defining a corresponding plurality of separately addressable electrodes; a first layer of white light emitting OLED material; a doped organic conductor layer; a second layer of white light emitting OLED material; and a second electrode layer defining a single electrode coextensive with the plurality of color filters. Similarly, US Publication 2005/0236981, filed Jun. 20, 2005, entitled “OLED Device” by Cok et al, which is hereby incorporated in its entirety by reference, describes an OLED device comprising a first layer of independently addressable light-emitting elements; and a second layer of independently addressable light-emitting elements located on top of the first layer; wherein one of the first and second layers of independently addressable light-emitting elements comprises a patterned array of red and blue light-emitting elements, and the other of the first and second layers of independently addressable light-emitting elements comprises an array of green light emitting elements. However, neither of these disclosures describes a drive circuit useful for controlling a stacked OLED device.
There is a need therefore for an improved control circuit for stacked organic light-emitting diode devices.